1. Field of Invention
This invention relates generally to wireless acoustic stimulation systems, devices, and methods for stimulating biological tissue and, in particular, for a receiver-stimulator that converts acoustic energy received from a controller-transmitter into electrical energy, and delivers the electrical energy to the tissue at a level that does not stimulate tissue during diagnostic echocardiography. Furthermore, this invention relates to a controller-transmitter that can determine the electrical energy output by a receiver-stimulator.
2. Description of the Background Art
Stimulation of cardiac tissue can be achieved using acoustic energy based systems comprising a controller-transmitter and one or more implanted receiver-stimulator devices. The controller-transmitter transmits acoustic energy by producing an acoustic field that is transmitted over time. The acoustic field is a propagating acoustic wave defined by its direction and its intensity (i.e., its power per unit area, typically expressed as W/m2). The acoustic field varies and attenuates as it propagates through the body due to absorption, refraction, and reflection. To minimize losses, the controller-transmitter focuses, or attempts to maximize, the acoustic field on the receiver-stimulator. In turn, the receiver-stimulator maximizes harvesting and converting of the acoustic field impinging upon it into electrical power delivered over time to the tissue to stimulate the tissue (stimulation energy). In general, this receiver-stimulator is a specialized transducer, that is, a device that converts acoustic energy to electrical energy. In another perspective the receiver-stimulator uses the converted energy as a tissue stimulator that delivers electrical energy to cardiac or other tissue through tissue stimulation electrodes. The controller-transmitter may be applied externally on the body, but will usually be implanted in the body, requiring that the controller-transmitter have a reasonable size, similar to that of implantable pacemakers, and that the controller-transmitter be capable of operating from batteries for a lengthy period, typically three or more years. Thus, a chronically implanted acoustic wireless stimulation system that uses ultrasound to transfer pacing energy requires all aspects of the system to be as energy efficient as possible to increase the service life of the device and additionally reduce the battery size.
Furthermore, it is also conceivable that a recipient of the implanted acoustic wireless stimulation system may be exposed to environments with interfering acoustic fields, such as acoustic fields generated by diagnostic ultrasound imaging devices. Interference, defined as inadvertent pacing/stimulation from diagnostic imaging systems, is a particular concern when designing a wireless system that maximizes acoustic to electrical energy conversion efficiency. The FDA guidance on diagnostic ultrasound imaging (2008) provides the following acoustic output limits for track 1 devices (track 1 devices do not require specific labeling for large acoustic output):
TABLE 1UseISPTA.3 (mW/cm2)ISPPA.3 (W/cm2) or MIPeripheral Vessel7201901.9Cardiac4301901.9Fetal Imaging & Other*941901.9Ophthalmic17280.23
The relevant limit of concern to the present invention is the ISPPA limit which is presented as the acoustic intensity averaged over a single imaging pulse taken at the spatial peak of the field. The spatial peak is the point in space where the largest acoustic field is produced. According to table 1, the acoustic output limit for a track 1 diagnostic device is 190 W/cm2 (1.9E6 W/m2). A typical acoustic field required to pace the heart with the implanted receiver-stimulator would be on the order of 170 W/m2, which is 11,000 times smaller than the track 1 diagnostic device limit of 1.9E6 W/m2. In the context of the wireless stimulator, this implies that imaging devices will be producing acoustic fields that are very large (four orders of magnitude larger) than those required to stimulate cardiac tissue.
In addition to the strength of the acoustic fields, the operation frequency and duration of pulses used in diagnostic imaging must also be taken into account to determine any potential acoustic interference from diagnostic devices.
Most diagnostic imaging systems operate within the frequency range of 2-10 MHz, whereas the typical operating frequency for an implanted acoustic wireless system is within the range of 800 kHz to 1.3 MHz. Although most diagnostic imaging systems operate at higher frequencies than the implanted acoustic wireless system, state of the art systems can operate in a harmonic imaging mode where transmission occurs at a lower frequency and reception intentionally occurs at a harmonic multiple of the transmit frequency. For example, the Phillips system transmits at 1.2 MHz but receives at 2.4 MHz. It is also known that some diagnostic imaging systems in use today can operate at as low as 500 KHz in the harmonic imaging mode. Therefore, it is reasonable to assume that at least some diagnostic imaging systems operate with sufficient acoustic intensity and within a frequency range that may interfere with the operation of implantable acoustic stimulation systems without any safeguard measures.
To achieve optimal stimulation energy efficiency, an implanted acoustic wireless stimulation system should receive acoustic energy at a pulse-width within the range of 0.2-1.0 ms. Pulses that are shorter or longer than the optimal range require more stimulation energy than a pulse that is within the optimal range.
FIG. 1 illustrates the electrical energy needed to stimulate the tissue at different pulse widths taken at different times during an acute procedure. The electrical energy required for sufficient pacing for the shortest duration pulse (0.02 msec=20 μsec) shown in FIG. 1 is 5-10 times greater than the minimum electrical energy required for an optimal pulse width of 500 μsec. The longest duration pulses used in diagnostic imaging are those used for Doppler modes and they can be up to 8 μsec long (for the purposes of illustration, duration pulses used in diagnostic imaging are approximated as 10 μsec long), which is reasonably close to the 20 μsec data points in the curves shown in FIG. 1. Taking into consideration that the duration is reduced from 20 μsec down to 10 μsec, it is reasonable then to infer that pulse widths used in diagnostic imaging would require approximately 20 times more converted acoustic energy to pace tissue than the converted acoustic energy of the optimal pulse widths used for wireless pacing. However the acoustic energy must be compressed into a 10 μsec pulse rather than the optimal pulse of 500 μsec. Therefore, the acoustic intensity required to stimulate the tissue with a short duration 10 μsec must be
                    20        ·        500            ⁢                          ⁢      µ      ⁢                          ⁢      sec              10      ⁢                          ⁢      µ      ⁢                          ⁢      sec        =      1    ,    000  times greater than that is required to stimulate with a 500 μsec pulse. This is three orders of magnitude, but is still insufficient protection as it does not provide the four orders of magnitude discussed previously that would be required to ensure that interference from diagnostic imaging sources does not stimulate cardiac tissue. Clearly, without some additional measures an implanted acoustic wireless stimulation system cannot be both optimally energy efficient by using the optimal pulse-width and immune to undesired stimulation in the presence of interfering acoustic fields such as those generated by diagnostic imaging equipment.
It would be desirable to provide implantable receiver devices which are able to function safely in the presence of interfering acoustic fields by limiting the converted electrical energy delivered to the tissue to a level that prevents undesired stimulation caused by any interfering acoustic fields.
The following patents and patent publications describe various wireless implantable stimulation systems: U.S. Pat. No. 7,283,874; U.S. Patent Publication Nos. 20070282383A1 and 20070233200A1. U.S. Pat. No. 7,283,874 by Penner et al. describes an implantable stimulation system that transmits acoustic waves to an implantable stimulator, and the implantable stimulator transforms the acoustic energy into electrical energy. The patent discloses using a voltage protector to prevent the energy storage device from overcharging, however the patent does not address preventing undesired stimulation due to acoustic interference from, say, an imaging system.
Historically, the very large body of knowledge associated with the assessment of cardiac tissue stimulation is based on delivery of electrical energy through electrodes of a wire/lead based system. The amount of energy required to stimulate tissue, referred to as the threshold, is affected by several typical system factors, e.g. electrode surface area, current density at the electrode-tissue interface, distance of an electrode from excitable tissue, etc. The threshold is expressed in electrical terms. Stimulation threshold is achieved when sufficient amplitude (voltage) delivered for sufficient time (pulse width) through an electrode-tissue interface (impedance) activates/stimulates tissue (capture). Practitioners of cardiac pacing identify the stimulation threshold for several practical reasons. One reason is to ensure that the pacing electrode is in proximity of excitable/viable tissue in order to select a suitable implant location and in order to use a reasonable energy level from a battery-based pacing system, a pacemaker. A low stimulation threshold, for example 1 Volt at 0.5 ms, would indicate that the electrode location would be adequate for acute and chronic pacing. Another reason that the stimulation threshold is identified is so that pacing systems can be programmed to output voltages and pulse width durations that exceed the threshold by a safety factor. Typically a 2-3 times threshold is used as the selected/programmed settings in a pacemaker. With this knowledge base in wide historic usage, it is highly desirable to be able to quantify the electrical energy being delivered to tissue by a wireless pacing system.
Current wireless implantable stimulation systems are unable to detect or measure the pacing voltage that is actually applied to the tissue, because there is no direct electrical connection to the implantable receiver electrode. Therefore, it would also be desirable to provide systems, devices and methods to infer the pacing voltage applied to the tissue in wireless implantable stimulation systems. It would also be desirable to have the implantable receiver devices limit the electrical energy output to a level that prevents interference from diagnostic acoustic sources. At least some of these objectives will be met by the inventions described hereinafter.